Analog and mixed signal circuits typically use analog switches that include transistors such as MOSFETs as switching elements. Analog switches are usually driven by a DC control signal that can switch the MOSFETs either on or off. When the DC control signal is applied to the gate of a MOSFET, the signal reduces the ON resistance (Ron) of the MOSFET, thereby allowing the MOSFET to send an analog signal from the source to the drain.
Resistance Ron of a MOSFET in an analog circuit is a factor in maintaining a high degree of linearity in the circuit. Fluctuations in gate to source voltage (Vgs) can modulate the resistance Ron of the MOSFET, which in turn can distort signals passing through the MOSFET. For example, fluctuations of Ron can occur, due to bulk-effect (i.e., modulation of the MOSFET threshold-voltage, due to variations of its source-to-bulk potential), or if the source of the MOSFET is not connected to an AC ground (i.e., the source signal contains AC components) while the gate of the MOSFET is driven by a DC control signal; or when when the gate voltage of the MOSFET fluctuates, and the source is tied to a DC voltage.
FIG. 1 is a prior art differential gain stage 100 with programmable input and feedback resistances. The differential gain stage 100 includes a differential input signal VinD 102 and a common mode input signal VinCM 104. The input signals are added at summing devices or adders 106, and the added signals are fed to the input terminals of an operational amplifier (op-amp) 108. The path of the added signals includes analog switches 110-1 and 110-2 implemented as MOSFETs, and programmable resistors 112-1 and 112-2. The op-amp 108 further includes two negative feedback loops having analog switches 110-3 and 110-4 implemented as MOSFETs, and programmable resistors 112-3 and 112-4. Nodes Vn 114 and Vp 116 represent virtual ground nodes of the op-amp 108 and follow the common mode input signal VinCM 104, with a factor of: 1/(1+(R 112-1/R 112-3)), since this voltage swing is not suppressed by the op-amp loop-gain, as with the differential signal.
The common mode input voltage VinCM 104 can have a non-zero AC component, or noise, that is reproduced at virtual ground nodes Vn 114 and Vp 116. In addition, as the source terminals of the MOSFET switches 110 are connected to the virtual ground nodes Vn 114 and Vp 116, the AC component at the virtual ground nodes Vn 114 and Vp 116 modulates the resistance Ron of the switches. This introduces fluctuations in the MOSFET switches 110, which in turn cause distortions in output signals Voutp 118 and Voutn 120.
Active devices such as filters can be employed to suppress the AC component of the common mode input signal VinCM 104; however, filters themselves can introduce noise into a circuit. Another method used for reducing the distortion introduced by MOSFET switches is to reduce the resistance Ron of the switches by increasing the size of the switches. Such an arrangement can lead to higher area consumption and an increased MOSFET parasitic capacitance. Another way to reduce Ron is by increasing the gate to source volatge Vgs; however, Vgs is limited by the supply voltage Vdd 122. Therefore, there is typically noise or fluctuations in the common mode input signal which cause distortions in signals passing through the MOSFET switches 110, and can distort the output signals Voutp 118 and Voutn 120.